The following primary methods of producing super-lightweight ceramic foams are known in the art.
Impregnating a polyurethane foam having continuous cells with a slurry containing, among other things, ceramic powder and an organic binder, drying and heating the impregnated foam to remove the organic component therefrom, and sintering the foam.
Mixing a foamable polyurethane material with a slurry containing, among other things, ceramic powder and an organic binder, foaming the mixture, solidifying the foam, heating the solidified foam to remove the organic component therefrom, and sintering the foam.
Adding a surfactant into a dispersion medium containing ceramic powder and a hydrophobic resin binder to emulsify the hydrophobic resin binder therein, foaming the emulsion, solidifying the foam, heating the solidified foam to remove the organic component therefrom, and sintering the foam.
Adding a filler, such as plastic beads, into a slurry, wherein the filler is to be thermally removed in a subsequent firing process in order to form cells, drying the slurry, and firing the dried body to produce a material having a foam-like structure.
Also, the present inventors have previously proposed a method of producing a lightweight ceramic material, comprising adding a given amount of inorganic power into a diluted aluminum-hydroxide sol solution serving as a binder, foaming the solution, drying the foam, and firing the foam, as described in Japanese Patent Laid-Opening Publication No. 2002-114584, which is incorporated herein by reference in its entirety.
In the above-described methods 1-4 of producing ceramic foams, a large amount of organic matter is commonly added as a binder or filler. Thus, methods 1-4 inevitably involve a problem of cracks and/or fractures possibly caused by gases generated in the process of thermally removing the organic matter in advance of sintering the ceramic powder, or the difference in thermal expansion between the organic and ceramic phases. In particular, a lightweight ceramic foam is formed having a cellular structure with a thin cell wall having poor strength, and thus the above-described adverse effect leads to more serious defects, such as deteriorated strength or powdering due to micro-cracks caused in the sintering process. While this problem may be solved by providing enhanced strength in the cellular structure without increasing the amount of the binder, the cell wall of the cellular structure inevitably becomes thicker, resulting in lowered porosity and increased density in a ceramic foam to be obtained.
A lightweight ceramic foam produced through the above-described method using a diluted aluminum-hydroxide sol solution also has a low-strength cellular structure, and further involves a problem of limited uses due to powdering caused by scratching or scraping the ceramic foam.
Thus, when a ceramic foam is produced with a higher porosity to facilitate reduction in the weight thereof, the strength of its cellular structure is reduced accordingly and must be compensated for without an increase in the density of the ceramic foam. In this connection, the inventors are not aware of any attempt to disperse ceramic short fibers in a ceramic foam to achieve the reinforcement of its cellular structure.
Monolithic ceramics reinforced by ceramic short fibers dispersed therein are known. In a process of producing such monolithic ceramics, the ceramic short fibers are added into a raw material slurry directly or after having been mixed with a dispersion medium containing any suitable type of surfactant, and the slurry is simply stirred using a mechanical blender, such as a ball mill or attritor mill, to facilitate dispersion of the ceramic short fibers throughout the slurry.
However, the ceramic short fibers generally become entangled with each other and agglomerate, and the above-described dispersion method does not allow such ceramic short fibers to be evenly dispersed throughout the slurry. Particularly in the process of dispersing ceramic short fibers in a lightweight ceramic foam, agglomerates of short fibers cause a critical defect by damaging the thin cell wall of the foam's cellular structure. In addition, if the slurry is stirred in the blender for a long time, the fibers will be inevitably damaged by media balls in the blender. The intended purpose of dispersing ceramic short fibers in the ceramic matrix is to reinforce the cellular structure or provide a higher toughness to the cellular structure by utilizing the toughness of the ceramic short fiber. If the fiber itself is damaged in the dispersion process, the expected effect will be significantly deteriorated.